A gas turbine engine extracts energy from a flow of hot gas produced by combustion of gas or fuel oil in a stream of compressed air. In its simplest form, a gas turbine engine has an air compressor (radial or axial flow) fluidly coupled to a turbine with a combustion chamber disposed therebetween. Energy is released and work is performed when compressed air is mixed with fuel and ignited in the combustor, directed over the turbine's blades, spinning the turbine. Energy is extracted in the form of shaft power (e.g., turboshaft engines) and/or compressed air and thrust (e.g., turbojet/turbofan engines).
Irrespective of the exact engine type, most gas turbine engines operate in a similar manner. Initially, ambient air is received at the inlet of the compressor where it is compressed and discharged at a substantially higher pressure and temperature. The compressed air then passes through the combustion chamber, where it is mixed with fuel and burned thereby further increasing the temperature and, by confining the volume, the resultant pressure for combustion gases. The hot combustion gases are then passed through the hot turbine section where mechanical shaft power may be extracted to drive a shaft, propeller or fan. Any remaining exhaust gas pressure above ambient pressure can be used to provide thrust if exhausted in rearward direction.
Some turbine engines also try to recover heat from the exhaust, which otherwise is wasted energy. For instance, a recuperator is often used in association with the combustion portion of a gas turbine engine, to increase its overall efficiency. Specifically, the recuperator is a heat exchanger that transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel combustor stage. Since the compressed air has been preheated, less fuel is needed to heat the compressed air/fuel mixture up to the turbine inlet temperature. By recovering some of the energy usually lost as waste heat, the recuperator can make a gas turbine significantly more efficient.
Use of a recuperator, while improving efficiency of a gas turbine engine, can also have a number of disadvantages in various applications. One such potential disadvantage is the reduction of power of a turbine engine that includes a recuperator. As may be appreciated, passing compressed air from the compressor through plumbing associated with a recuperator/heat exchanger results in a pressure drop of the compressed air thereby reducing the high-end performance (e.g., maximum power) of the engine. Such reduced power output is especially disadvantageous in aircraft and helicopter applications where maximum power is often desired and/or necessary during takeoff or hot and high altitude flying.
Another potential disadvantage is the increased weight of a turbine engine incorporating a recuperator. Such a disadvantage is also evident in aircraft applications where turbine engines are often utilized due to their high power to weight ratio. That is, in most cases, gas turbine engines are considerably smaller and lighter than reciprocating engines of the same power rating. For this reason, turboshaft engines are used to power almost all modern helicopters. Typically, incorporation of a recuperator has heretofore resulted in significant addition of weight to the turbine engine. Historically, the added weight and cost of the recuperator and associated system plumbing has more than offset any reduced fuel consumption, yielding endurance break-even times that are much too long for typical flight times.
For at least these reasons, use of recuperators have not found widespread acceptance in the light aircraft and helicopter industry.